The term "higher order hydrocarbon" refers to a hydrocarbon product having at least one more carbon atom in its structure than the hydrocarbon used in the reactant to form said higher order hydrocarbon. For example, if the reactant comprised methane, and ethane was formed from said reactant, ethane would be a higher order hydrocarbon. The term "substantially liquid hydrocarbon" refers to hydrocarbons that are substantially in the liquid form at a temperature of about 25.degree. C. and a pressure of one atmosphere.
The production of acetylene by the cracking of petroleum hydrocarbons with steam or the partial oxidation of natural gas is known. The production of ethylene by the thermal cracking of ethane, propane, butane, naphtha and refinery off-gases is known.
U.S. Pat. No. 3,093,697 discloses a process for making acetylene by heating a mixture of hydrogen and a hydrocarbon stock (e.g., methane) at a reaction temperature that is dependent upon the particular hydrocarbon employed for about 0.01 to 0.05 second. The reference indicates that a reaction temperature of 2700.degree. F. (1482.degree. C.) to 2800.degree. F. (1538.degree. C.) is preferred for methane and that lower temperatures are preferred for higher molecular weight hydrocarbons.
U.S. Pat. No. 3,156,733 discloses a process for the pyrolysis of methane to acetylene and hydrogen. The process involves heating a methane-containing stream in a pyrolytic reaction zone at a maximum temperature above 1500.degree. C. and sequentially withdrawing a gaseous product from said reaction zone and quenching said product rapidly to a temperature of about 600.degree. C. or less.
U.S. Pat. No. 4,176,045 discloses a process for the production of olefins by steam-cracking normally liquid hydrocarbons in a tubular reactor wherein the residence time in the tubes is from about 0.02 to about 0.2 second and the formation of coke deposits in the tubular reactor is minimized.
Chemical Economy and Engineering Review, July/August 1985, Vol. 17, No. 7.8 (No. 190), pp. 47-48, discloses that furnaces have been developed commercially for steam cracking a wide range of liquid hydrocarbon feedstocks using process reaction times in the range of 0.05 to 0.1 second. This publication indicates that the use of these furnaces permits substantial increases in the yield of olefins (i.e., ethylene, propylene, butadiene) while decreasing production of less-desirable co-products.
Natural gas typically contains about 40-95% methane depending on the particular source. Other constituents include about 10% of ethane with the balance being made up of CO.sub.2 and smaller amounts of propane, the butanes, the pentanes, nitrogen, etc. Primary sources for natural gas are the porous reservoirs generally associated with crude oil reserves. From these sources come most of the natural gas used for heating purposes. Quantities of natural gas are also known to be present in coal deposits and are by-products of crude oil refinery processes and bacterial decomposition of organic matter. Prior to commercial use, natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature and then washing the natural gas with a cold hydrocarbon liquid to absorb the condensible hydrocarbons. The condensible hydrocarbons are typically ethane and heavier hydrocarbons. This gas processing can occur at the wellhead or at a central processing station. Processed natural gas typically comprises a major amount of methane, and minor amounts of ethane, propane, the butanes, the pentanes, carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 70% to more than about 95% by volume of methane. Natural gas is used principally as a source of heat in residential, commercial and industrial service.
Most processed natural gas is distributed through extensive pipeline distribution networks. As natural gas reserves in close proximity to gas usage decrease, new sources that are more distant require additional transportation. Many of these distant sources are not, however, amenable to transport by pipeline. For example, sources that are located in areas requiring economically unfeasible pipeline networks or in areas requiring transport across large bodies of water are not amendable to transport by pipeline. This problem has been addressed in several ways.
One approach has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. This approach is limited as the natural gas can be used only for one product, preempting other feasible uses. Another approach has been to liquefy the natural gas using cryogenic techniques and transport the liquid natural gas in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by such cryogenic processing, and with proper procedures, safely stored or transported. These processes, which involve liquefying natural gas to a temperature of about -162.degree. C., transporting the gas, and revaporizing it are complex and energy intensive.
Still another approach involves the use of pyrolysis to convert the natural gas to higher order hydrocarbons (e.g., substantially liquid hydrocarbons) that can be easily handled and transported. Low temperature pyrolysis (e.g., to 250.degree. C. and 500.degree. C.) is described in U.S. Pat. Nos. 4,433,192; 4,497,970; and 4,513,164. The processes described in these patents utilize heterogeneous systems and solid acid catalysts. In addition to the solid acid catalysts, the reaction mixtures described in the '970 and '164 patents include oxidizing agents. Among the oxidizing agents disclosed are air, O.sub.2 -O.sub.3 mixtures, S, Se, SO.sub.3, N.sub.2 O, NO, NO.sub.3, F, etc. The conversion of natural gas to higher order hydrocarbons at higher temperatures (e.g., above about 1200.degree. C.) using pyrolysis has been suggested. These high-temperature processes are, however, energy intensive and have thus far not been developed to the point where high yields are obtained even with the use of catalysts. Some catalysts that are useful in these processes (e.g., chlorine) are corrosive under such operating conditions.
A common technique for pyrolyzing natural gas involves the use of tubular reactors. The natural gas flows through a tube placed inside a radiant and/or convective chamber of a furnace. The heat supplied to the natural gas is dependent upon the surface area of the tubes, and thus only relatively small diameter tubes are typically used. When using such tubular reactors in the pyrolysis of natural gas, coke tends to build up on the inner walls of the tubes. Because of the small diameter of the tubes, any deposited coke forms a relatively thick layer and thereby severely inhibits further heat transfer. Tubular reactors can be used for cracking hydrocarbons like ethane or propane due to the fact that hydrocarbons of this type do not produce significant levels of coke. However, the amount of coke produced during pyrolysis of natural gas is substantially greater and thus tubular reactors cannot be operated continually for more than a few minutes or a few hours at a time. The use of tubular reactors for the pyrolysis of natural gas is also restricted by the materials available for making the tubes. Typically these reactors have maximum operating temperatures of only about 1050.degree. C. due to the materials of construction used in the tubes, while on the other hand significantly improved yields in the pyrolysis of natural gas could be achieved if higher operating temperatures could be used.
Riser reactors of the type disclosed in U.S. Pat. No. 4,061,562 have been suggested for thermal cracking of petroleum oils. A mixture of hot solids (e.g., non-catalytic alumina, alundum, carborundum, coke, etc.), feed oil (e.g., hydrodesulfurized residual petroleum oil) and gaseous diluent (e.g., steam) flow co-currently through a thermariser at an average riser temperature of 1300.degree. F. to 2500.degree. F. (704.degree. C. to 1371.degree. C.) to produce hydrocarbon products. The hydrocarbon products obtained by this process include ethane, ethylene, propylene, 1,3-butadiene, other C.sub.4 hydrocarbons, benzene, toluene, xylene, liquids boiling in the gasoline range, and light and heavy gas oils. This patent indicates that during the process coke forms on the solids and is carried out of the reactor with such solids thus limiting harmful build-up of coke on the reactor walls. A disadvantage with these riser reactors is that due to the co-current flow of the solids and gases, such solids and gases must be separated by an external cyclone. Also, the product gases can only be quenched after separating the solids, otherwise the process would be thermally inefficient. Another disadvantage is that it is not possible to use relatively short residence times (e.g., below about 50 milliseconds) and thus the use of these reactors for pyrolyzing feedstocks such as natural gas is severly limited.
Fluidized beds typically comprise a processing chamber which is partially filled with particulate solids. The floor of the chamber constitutes a perforated plate and, in use, a gas is forced up through this plate. The particulate solids in the chamber are agitated sufficiently so as to form a turbulent mass resembling a boiling liquid. This is the "fluidized bed". Heating of the fluidized bed can be effected by combustion of the gas below the plate before it enters the chamber, or by internal combustion of the gas within the bed. In theory, the fluidized bed provides an effective transfer mechanism which offers benefits in a variety of thermal and/or catalytic processing systems. In practice, however, the use of fluidized beds has been limited when relatively high heat and/or mass transfer rates have been desired due to, among other things, undesirable entrainment of the solid particles out of the fluidized bed. As the demand for higher heat and/or mass transfer rates has evolved, means have been sought to increase the performance of fluidized bed processes. This has usually led to the use of higher fluidizing velocities resulting in the use of "entrained beds". The use of entrained beds has not, however, been entirely satisfactory. One of the problems with the use of such entrained beds relates to the inherent problems involved with recovering and recirculating entrained particulate solids.
U.S. Pat. Nos. 4,479,920 and 4,559,719 disclose an apparatus and process for processing matter in a turbulent mass of particulate material in a substantially annular processing region. These patents indicate that the processing region is preferably in the form of a substantially annular processing chamber having a radially inner wall which includes a waist. A flow of fluid and particulate material to be processed are admitted to the processing region through one or more inlets with the flow of fluid being directed generally circumferentially into the processing region. In the processing region, particulate material to be processed is embedded in a compact turbulent band. Once processing is complete, the processed matter is withdrawn from the processing region, preferably by entrainment in an exhaust flow of the fluid.
Combustion is an exothermic oxidation reaction in which the heat evolved results from the formation of carbon oxygen bonds. For each carbonaceous material there is a specific carbon to oxygen ratio that ideally corresponds to complete or stoichiometric combustion. The terms "complete combustion" and "stoichiometric combustion" are used herein to refer to the conversion of a carbonaceous material to CO.sub.2 with no CO being produced. Complete combustion is often desirable because the production of heat is maximized and the production of pollutants (e.g., carbon monoxide) is eliminated. Complete or stoichiometric combustion of methane, acetylene, ethylene, ethane, propane, butane and benzene can be represented by the following equations: EQU CH.sub.4 +2O.sub.2 =CO.sub.2 +2H.sub.2 O EQU C.sub.2 H.sub.2 +2.5O.sub.2 =2CO.sub.2 +H.sub.2 O EQU C.sub.2 H.sub.4 +3O.sub.2 =2CO.sub.2 +2H.sub.2 O EQU C.sub.2 H.sub.6 +3.5O.sub.2 =2CO.sub.2 +3H.sub.2 O EQU C.sub.3 H.sub.8 +5O.sub.2 =3CO.sub.2 +4H.sub.2 O EQU C.sub.4 H.sub.10 +6.5O.sub.2 =4CO.sub.2 +5H.sub.2 O EQU C.sub.6 H.sub.6 +7.5O.sub.2 =6CO.sub.2 +3H.sub.2 O
In the above equations, 2 moles of oxygen are required per mole of methane to achieve complete or stoichiometric combustion of methane; 2.5 moles of oxygen are needed per mole of acetylene to achieve complete or stoichiometric combustion of acetylene; 3 moles of oxygen per mole of ethylene are needed to achieve complete or stoichiometric combustion of ethylene; etc. Similar equations can be used to represent the complete or stoichiometric combustion of other carbonaceous materials. The amount of air required for a stoichiometric mixture for many carbonaceous fuels is provided in Perry, J. H., et al, Editors, "Chemical Engineer's Handbook", Fourth Edition (1963) at pp. 9-31 to 9-33.
In large-scale or commercial operations involving the combustion of carbonaceous fuels, it is usually not possible to obtain complete combustion with only a stoichiometric amount of oxygen or air. It is thus common practice to add excess oxygen or air (that is, oxygen or air in excess of stoichiometric amount required to provide complete combustion) to effect complete or substantially complete combustion. The amount of excess oxygen or air required depends on many factors including the particular carbonaceous fuel being burned, the type of burner or furnace being used, etc. Adding excess oxygen or air has certain disadvantages. The excess oxygen or air decreases the efficiency of the combustion process by reducing its ultimate obtainable temperature and by increasing the size of the equipment necessary to convert all of the carbonaceous fuel to carbon dioxide. The use of oxidation catalysts to enhance combustion and thereby eliminate the requirement for excess oxygen or air has been suggested.
U.S. Pat. Nos. 3,926,854 and 3,947,380 disclose ceramic mixed oxide, non-stoichiometric electrically neutral rare-earth-type catalysts containing rare-earth--type elements and elements of the first transition metal series and optionally alkaline earth metals. These catalysts have the following formula: EQU X.sub.n J(1-n)ZO(3+m)
wherein: X is an alkaline earth metal or mixture thereof; J is a rare-earth-type element or mixture thereof; Z is a metal of the first transition series or a mixture thereof, at least 0.01% of said metal having an oxidation state other than +3; m is a number having a value of between zero and about 0.11; and n is a number having a value from zero to about 0.51. These patents indicated that these catalysts can be used to catalytically oxidize low molecular weight inorganic compounds and elements, such as ammonia, carbon monoxide, hydrogen, sulfur dioxide, and hydrogen sulfide, with oxygen, or carbon monoxide with water, sulfur dioxide or nitric oxide. The catalyst can also be employed in the catalytic removal of carbon monoxide, hydrocarbons and nitric oxides from the exhaust gases of generating or heating plants and automobiles burning fossil fuels.
U.S. Pat. Nos. 3,885,020; 3,976,599; 4,076,486; 4,124,689; and 4,124,697 disclose ceramic mixed oxide, non-stoichiometric electrically neutral, rare-earth-type catalysts containing rare-earth-type elements, elements of the first transition metal series and zirconium, tin or thorium and optionally alkaline earth metals. These catalysts have the following formula: EQU W.sub.k X.sub.n J(1-k-n)ZO(3+m)
wherein: W is zirconium, tin or thorium or mixture thereof; X is an alkaline earth metal or mixture thereof; J is a rare-earth-type element or mixture thereof; Z is a metal of the first transition series or a mixture thereof, at least 0.01% of said metal having an oxidation state other than +3; k is a number having a value of between zero and about 0.1; m is a number having a value of from zero to about 0.26, provided m has a value other than zero when n has a value of zero; and n is a number having a value from 0 to about 0.51 provided when n has a value of zero, k has a value of between zero and about 0.05. These patents indicate that these catalysts can be used to catalytically oxidize organic compounds to various states of oxidation, ammonia, carbon monoxide, hydrogen, sulfur dioxide, and hydrogen sulfide, with oxygen, or carbon monoxide with water, sulfur dioxide or nitric oxide. The catalyst can also be employed in the catalytic removal of carbon monoxide, hydrocarbons, nitric oxides and sulfur dioxide from the exhaust gases of generating or heating plants and automobiles burning fossil fuels. In addition, these catalysts can be employed to produce hydrogen cyanide from methane, ammonia and oxygen.